U.S. Pat. No. 5,244,845 (Kerko et al.) discloses transparent, substantially colorless photochromic glasses designed for use in the fabrication of conventional ophthalmic lenses (a thickness of about 2 mm) and in the fabrication of photochromic glass lens elements (a thickness of about 1 mm) for use in laminated glass-plastic, ophthalmic lens structures, which glasses darken to a specifically limited brown hue, depending upon the thickness of the glass, upon being subjected to actinic radiation.
As explained in that patent, the glass compositions disclosed were devised to constitute an improvement upon the glasses described in U.S. Pat. No. 4,608,349 (Kerko et al.), which glasses had been expressly designed for use as thin lens elements in laminated glass-plastic ophthalmic lens structures and consisted essentially, in weight percent, of
______________________________________ SiO.sub.2 54-58 ZrO.sub.2 2-4.5 B.sub.2 O.sub.3 18-22 Ag 0.2-0.33 Al.sub.2 O.sub.3 7-8 Cl 0.3-0.5 Li.sub.2 O 3.75-4.5 Br 0.04-0.12 Na.sub.2 O 0-1 CuO 0.007-0.012 K.sub.2 O 5.5-7.5 PbO 0-0.08 TiO.sub.2 0-2 Sb.sub.2 O.sub.3 0-0.2 ______________________________________
wherein the molar ratio Li.sub.2 O:Na.sub.2 O.gtoreq.9. The glasses were stated to exhibit a neutral gray tint in the darkened state and to be essentially colorless in the undarkened state, having a luminous transmittance over the range of about 88-92%.
The basic objective of U.S. Pat. No. 5,244,845 was to develop a thin photochromic lens of about 1 mm thickness capable of darkening to a brown coloration when subjected to actinic radiation, such as to be operable in the above-described laminated lens structure. The patent first acknowledged that a variety of additives was known which can be incorporated into photochromic glasses of specific compositions to develop a brown color therein. Those additives included Au, Pd, SnO, Fe.sub.2 O.sub.3, NiO, Er.sub.2 O.sub.3, Nd.sub.2 O.sub.3, and Co.sub.3 O.sub.4, the utility of those additives being disclosed in U.S. Pat. No. 4,251,278 (Hares), U.S. Pat. No. 4,757,034 (Prassas), and U.S. Pat. No. 5,023,209 (Grateau et al.). The glasses disclosed in those patents, however, were useful only in standard, 2 mm thick lenses. Thus, there was still the need for photochromic glasses which, at a thickness of about 1 mm, will darken sufficiently in a brown coloration to be operable as a thin glass lens element in the above-described laminated glass-plastic lenses.
That need was satisfied by utilizing the base glass compositions of U.S. Pat. No. 4,608,349, but modifying the Ag and Cl concentrations thereof and adding Pd with, optionally, Er.sub.2 O.sub.3 and/or Au. Accordingly, the glass compositions recorded in U.S. Pat. No. 5,244,845 consist essentially, in weight percent, of
______________________________________ SiO.sub.2 54-58 TiO.sub.2 0-2 PbO 0-0.08 B.sub.2 O.sub.3 18-22 ZrO.sub.2 2-4.5 Sb.sub.2 O.sub.3 0-0.2 Al.sub.2 O.sub.3 7-8 Ag 0.18-0.33 Er 0-1 Li.sub.2 O 3.75-4.5 Cl 0.25-0.5 Au 0-10 ppm Na.sub.2 O 0-1 Br 0.04-0.12 Pd 1-6 ppm K.sub.2 O 5.5-7.5 CuO 0.007-0.013 ______________________________________
wherein the mole ratio Li.sub.2 O:Na.sub.2 O.gtoreq.9.
Laboratory investigation and field experience indicated that not only were those glasses operable as thin glass elements, but also were capable of functioning as brown sunglasses in conventional ophthalmic lens thickness (2 mm).
Utilizing the standard light source, viz., Illuminant C, as defined in terms of the 1931 C.I.E. trichromatic colorimetric system, the chromaticity coordinates of those glasses fall within the trapezoid ABCDA in the drawing for a glass of 1.1 mm thickness, and within the trapezoid EFGHE for a glass of 2 mm thickness. The approximate coordinate values for the corner points of those two trapezoidal enclosures are tabulated below:
______________________________________ Point x y ______________________________________ A 0.3377 0.3337 B 0.3513 0.3422 C 0.3617 0.3315 D 0.3446 0.3265 E 0.3683 0.3478 F 0.3923 0.3613 G 0.4081 0.3452 H 0.3795 0.3365 ______________________________________
Interviews with potential customers for photochromic glasses exhibiting a brown coloration upon exposure to actinic radiation elicited the opinion that the brown coloration exhibited by the glasses of U.S. Pat. No. 5,244,845 at a thickness of about 2 mm was too reddish-brown in the darkened state. That is, a preference was shown for glasses having a chocolate brown coloration in the fully darkened state corresponding to the chromaticity coordinates that fall within the trapezoid IJKLI in the appended drawing.
Therefore, the primary purpose of the present invention was to devise glass compositions satisfying that preference.